diff --git a/src/Viewer/GraphicsWindowQt5.cpp b/src/Viewer/GraphicsWindowQt5.cpp
index eaadf30b7..ea1f650ce 100644
--- a/src/Viewer/GraphicsWindowQt5.cpp
+++ b/src/Viewer/GraphicsWindowQt5.cpp
@@ -409,7 +409,15 @@ bool GraphicsWindowQt5::init( Qt::WindowFlags f )
     if ( _ownsWidget )
     {
         _window->setTitle( _traits->windowName.c_str() );
-        _window->setPosition( _traits->x, _traits->y );
+        
+        // to get OS-dependant default positioning of the window (which is desirable),
+        // we must take care to only set the position if explicitly requested.
+        // hence we set X & Y to these marker values by default.
+        // And hence only set position if both are valid.
+        if ((_traits->x != std::numeric_limits<int>::max()) && (_traits->y != std::numeric_limits<int>::max())) {
+            _window->setPosition( _traits->x, _traits->y );
+        }
+        
         QSize sz(_traits->width, _traits->height);
         if ( !_traits->supportsResize ) {
           _window->setMinimumSize( sz );
@@ -419,6 +427,8 @@ bool GraphicsWindowQt5::init( Qt::WindowFlags f )
         }
         
         if (windowData->createFullscreen) {
+            // this doesn't seem to actually work, so we
+            // check the flag again in realizeImplementation()
             _window->setWindowState(Qt::WindowFullScreen);
         }
     }
diff --git a/src/Viewer/WindowBuilder.cxx b/src/Viewer/WindowBuilder.cxx
index 42ac30ac3..f4d3c3365 100644
--- a/src/Viewer/WindowBuilder.cxx
+++ b/src/Viewer/WindowBuilder.cxx
@@ -24,6 +24,7 @@
 #include <Main/fg_props.hxx>
 
 #include <sstream>
+#include <limits>
 
 #if defined(SG_MAC)
     #include <osgViewer/api/Cocoa/GraphicsWindowCocoa>
@@ -112,8 +113,10 @@ void WindowBuilder::makeDefaultTraits(bool stencil)
         traits->supportsResize = true;
         traits->width = fgGetInt("/sim/startup/xsize");
         traits->height = fgGetInt("/sim/startup/ysize");
-        traits->x = 50;
-        traits->y = 50;
+        
+        // these are marker values to tell GraphicsWindowQt5 to use default x/y
+        traits->x = std::numeric_limits<int>::max();
+        traits->y = std::numeric_limits<int>::max();
 #else
         SG_LOG(SG_VIEW,SG_ALERT,"requested Qt GraphicsWindow in non-Qt build");
 #endif